This invention relates to gravity measuring apparatus for use in a borehole, and in particular gravity measuring apparatus which is able to compensate for long term drift in measurements made by the apparatus.
Gravity measurements within boreholes are used to monitor fluid movements within a reservoir, such as oil displacement by water or liquid displacement by gas. Such fluid movements occur normally in oil and gas reservoirs as fluids are produced from or injected into wells. In many cases, it is desirable to control the movement of fluids within the reservoir by controlling production or injection rates to maximise producible reserves or to minimise production of water. However, to intelligently control the movement of reservoir fluids, it is necessary to measure parameters such as bulk formation density which are sensitive to the movement of these fluids.
However once the gravity measuring apparatus, or gravity meter, is downhole, it is inaccessible unless withdrawn from the borehole. Long term drift occurs in measurements made by gravity meters and whilst drift can be readily compensated for in the case of surface gravity meters, compensation is a particular problem when gravity meters are placed downhole. This is particularly so as to withstand conditions downhole, stability is often sacrificed for apparatus robustness which increases problems with long term drift.
It is one aim of the present invention to provide gravity measuring apparatus which can compensate for drift in measurements made when downhole, and which is sensitive to the movement of fluids within a reservoir.
In accordance with one aspect of the present invention there is provided a gravity measuring apparatus comprising gravitational force detection means including a test mass and which produces measurements related to gravitational force exerted on the test mass, characterised in that the gravitational force detection means is adapted for use downhole and includes compensation means to compensate for errors in measurements made whilst downhole.
By providing compensation means, measurements relating to the gravitational force can be corrected downhole for offset errors occurring within the apparatus.
By measuring the spatial gradient of gravity, offset errors can be compensated for. Thus preferably the compensation means may enable movement of the gravitational force detection means between at least two spaced apart positions to produce at least two measurements, and the compensation means may offset the two measurements against each other to compensate for errors.
Thus the compensation means may comprise a guide rail along which the gravitational force detection means is moveable over a calibrated distance. This is of particular advantage where the gravity measuring apparatus is removably positioned downhole on a wireline.
Where the test mass is biased by a biasing means, such as a spring, toward an equilibrium position about which measurements are made, compensation may occur by attaching the biasing means to the compensation means, with the compensation means being responsive to and compensating for changes in biasing force associated with the biasing means.
Preferably the test mass is attached to a first lever and is suspended between first electromagnetic field generating elements and the compensation means is attached to the first lever by the biasing means so as to compensate for changes in biasing force.
One form of compensation means may comprise a second lever and second electromagnetic field generating elements, wherein at least part of the second lever is positioned at an equilibrium position between the second electromagnetic field generating elements, and movements about the equilibrium position are used to compensate for changes in biasing force.
The first and second electromagnetic field generating elements desirably communicate with feedback means, with the feedback means altering the electromagnetic field generated by the first and second electromagnetic field generating elements so as to maintain the first and the second levers at their respective equilibrium positions, with signals produced by the second electromagnetic field generating elements being used by the feedback means to compensate for changes in biasing force.
Where the apparatus to be used permanently downhole, preferably the biasing means is made of non-magnetic steel for improved stability.
In accordance with another aspect of the invention, a further gravity measuring apparatus is provided, the apparatus comprising gravitational force detection means including a test mass and which produces measurements related to gravitational force exerted on a test mass, characterised in that the test mass is biased by a biasing means toward an equilibrium position about which measurements are made, and the biasing means is attached to compensation means to compensate for changes in biasing force associated with the biasing means.
The gravity measuring apparatus as aforesaid may be adapted for use permanently downhole, and may be used in a horizontal borehole.
The invention also lies in a method of installing a gravity measuring apparatus within a borehole, comprising the steps of incorporating a gravity measuring apparatus in drill pipe and placing the drill pipe down a borehole to complete a well for fluid production.